The case for sporadic cyanogenic glycoside evolution in plants

Cyanogenic glycosides are α-hydroxynitrile glucosides present in approximately 3000 different plant species. Upon tissue disruption, cyanogenic glycosides are hydrolyzed to release toxic hydrogen cyanide as a means of chemical defense. Over 100 different cyanogenic glycosides have been reported, with structural diversity dependent on the precursor amino acid, and subsequent modifications. Cyanogenic glycosides represent a prime example of sporadic metabolite evolution, with the metabolic trait arising multiple times throughout the plant lineage as evidenced by recruitment of different enzyme families for biosynthesis. Here, we review the latest developments within cyanogenic glycoside biosynthesis, and argue possible factors driving sporadic evolution including shared intermediates and crossovers with other metabolic pathways crossovers, and metabolite multifunctionality beyond chemical defense.